The phylogeny of this species is debated, with the black-throated loon and the Pacific loon traditionally being considered sister species, whereas a study using mitochondrial and nuclear intron DNA supported placing the black-throated loon sister to a clade consisting of the Pacific loon and the two sister species that are the common loon and the yellow-billed loon. In the former phylogeny, the split between the Pacific loon and the black-throated loon is proposed to have happened about 6.5 million years ago.[4]

Breeding adults are like a smaller sleeker version of common loon. They measure 58–74 cm (23–29 in) in length, 110–128 cm (43–50 in) in wingspan and weigh 1–2.5 kg (2.2–5.5 lb).[5] They have a grey head, black throat, white underparts and chequered black-and-white mantle. The black throat has purple reflections. Non-breeding plumage is drabber with the chin and foreneck white.[6] Its bill is grey or whitish and dagger-shaped.

In all plumages, lack of a white flank patch distinguishes this species from the otherwise very similar black-throated loon. The Pacific loon can be differentiated from the red-throated loon in winter by the latter's paler look, the fact that the red-throated loon has less of a contrast between both the crown and hindneck and the throat, and the bill that looks to be upturned.[6]

This bird makes a yodelling high-pitched wailing call on its breeding grounds, as well as harsh growls and barks, like its harsh kok-kok-kok-kok call.[6]

The Pacific loon breeds on tundra lakes, and winters in the open ocean or other large bodies of water. It breeds primarily in northern Canada and eastern Siberia, and winters along the Pacific coast of North America.[7]

The Pacific loon constructs its nest on the ground near deep lakes. This nest is made out of piled-up vegetation.[9]

This loon lays a clutch of one to two light buff or green eggs with brown spots of various sizes. These eggs typically measure 76 by 47 millimetres (3.0 by 1.9 in). Although the eggs are laid a few days a part, they can usually be seen to hatch at intervals not more than a day. Incubation lasts 23 to 25 days.[9]

This species, like all divers/loons, is a specialist fish-eater, catching its prey underwater. It also forages in groups, usually swimming under schools of fish and forcing them up towards the surface.[9] It does, although, generally feed closer to the shore than other loons.[6]